Jbenchley, goto hardware manager, device properties, advanced and look for speed/duplex-but I wouldn't encourage this option, I'm not sure what level of handshaking ps3 and psm have, but you're essentially only allowing your data to flow one direction at a time. If there is any error checking or sync signal I would expect handshaking probs/network errors.
and basically you shouldn't need to do it - I'm running windows 7 on P4 3GHz 4gig ram without any stutter even when running background tasks. I'd suggest checking your task manager to see where your bottleneck is - network or CPU (sorry if teaching you to suck eggs). With a gigabit network I find a 1080p mkv stream hardly rates, the cpu, however, can be working it. I did find I had a lot of stutter even on a dvdrip stream when I had firefox running with several windows open. When I closed that down even a 1080p DTS bluray stream ran sweet.
Someone earlier (Lee I think) asked if replacing a 10/100 switch with a giganet switch would improve their speeds - it depends a little how your network is setup. The 10/100 switch will bottleneck any giganet devices running through it. Saying that, if you've only got a 10/100 card in your PC then it won't make any difference.
Just reading your other post Lee and a bit unsure how you've got things setup, you mention a 10/100 switch in one post and a crossover cable in another? I might be mixing my posts.... if you're running through a 10/100 switch to your PS3, see above. If you're running from your computer (with a giganet card/port) to your ps3 using a crossover cable check how its being detected at each end, depending on the cable you're using it might be limiting you to 10/100. If you've got 10/100/1000 on each end, you might be able to run a standard straight through cable and get automatic crossover (http://en.wikipedia.org/wiki/Ethernet_crossover_cable
hope thats some help.